Servo motors are a type of control motor, and are characterized as having exceptional rotational speed control and positional control. Servo motors are installed in a variety of work machines, and in particular are widely employed as driving sources for robots.
Servo motors are capable of directly driving a load. However, the load is often driven via a reducer or other power transmission mechanism. This is due to the fact that the output torque from the servo motor can be markedly increased when the load is driven via a reducer. In addition, if the reducer is composed of a belt, chain, or drive shaft, a benefit will be presented in that the movement can be transmitted to a location that is distant from the servo motor.
The fact that faults occur in servo motors and power transmission mechanisms in a work machine must be taken into account.
Faults that occur in work machines range from serious faults, which are critical enough to necessitate an emergency shutdown of the machine; minor faults, which cause no interference even if operation is continued until the time of the next repair; and moderate faults, which are between the serious faults and the minor faults in terms of severity.
The presence of faults in the servo motor itself can be determined by detecting anomalies via monitoring electric current values or performing other electrical monitoring. On the other hand, anomalies are difficult to detect when the servo monitor is operating normally but a fault has occurred in the power transmission mechanism. A detecting technique such as disclosed in JP-5-346812 A and JP-11-129186 A has been required.
In the anomaly-detecting device disclosed in JP-5-346812 A, a post-feedback command signal is integrated when the servo motor is being controlled by a controlling device via feedback. The time required for the resulting integration value to reach saturation is monitored. For example, the servo motor drives an arm as a load via a belt. However, if the belt breaks, the load suddenly changes, and the integration value changes markedly. Specifically, the load decreases, which causes fluctuations in the load to become less pronounced and the magnitude of the post-feedback command signal to decrease. When this occurs, the time required for the integration value to reach saturation (saturation time) increases. If a configuration is used in which an anomaly is determined when the saturation time extends past a reference time, the breaking of a belt or another anomaly can be detected. However, an anomaly cannot be detected if the change in the load fluctuation is small. In other words, the anomaly detecting device disclosed in JP-5-346812 A is intended for discovering serious faults such as the breaking of a belt, and is unable to address minor faults.
In the work robot disclosed in JP-11-129186 A, the work rate W1 on the drive side of a driving shaft is calculated on the basis of a driving current Ii and actual angle θi of a servo motor. A work rate W0 on the load side of the driving shafts is additionally calculated on the basis of the actual angle θi and a motion equation that relates to the particle model of a robot mechanism part. The difference or ratio between the resulting work rate W1 and the work rate W0 is compared with an established criterion. If a gear or the like is worn down due to change over time, the work rate W1 will generally be greater than the work rate W0 during acceleration. The difference or ratio at this time will become progressively more pronounced as the wear on the gear or other components increases. In other words, a detectable difference or ratio will not be generated in the event of small-scale wear. This approach is suitable for serious faults in which a difference occurs between the work rates, but is unsuitable for minor faults in which a difference between the work rates is not likely to occur.
A first problem arises in that a monitoring technique that corresponds to minor faults is not established in JP-5-346812 A or JP-11-129186 A.
It is also possible that, e.g., a turntable and a robot occupy a single work area. The turntable, in which a servo motor is used as a driving source, would be monitored by the anomaly-detecting device disclosed in JP-5-346812 A, and the robot would be monitored by the anomaly sensor of the work robot disclosed in JP-11-129186 A. Faults will be detected in the turntable and robot using different fault monitoring systems.
In other words, when there are multiple types of work machines manufactured by different makers, the fault monitoring systems attached to the servo motors for driving the work machines tend to have various functions. In such instances, diverse fault monitoring systems must be arranged in the work area (the production site or the surrounding area), substantial effort is required to maintain control over the systems, and greater space is needed to accommodate the systems.
In other words, the second problem arises from the fact that no technique has been established for allowing central control over a variety of servo motors when a variety of work machines are present.
A demand has accordingly arisen for a technique whereby a monitoring technique corresponding to minor faults can be performed and a variety of servo motors can be centrally controlled.